# The Centrocone Protein SMC_N1 Is Important for the Proliferation of Toxoplasma gondii Tachyzoites

## TL;DR

This study shows that the SMC_N1 protein is essential for the reproduction of the Toxoplasma gondii parasite, which could help in developing new treatments.

## Contribution

The study identifies SMC_N1 as a key protein in T. gondii proliferation and reveals its role in organelle inheritance and cell division.

## Key findings

- SMC_N1 is periodically expressed, peaking during the S phase and absent in the G1 phase.
- Depletion of SMC_N1 disrupts IMC assembly, endodyogeny, and nuclear division.
- SMC_N1 is involved in the stable inheritance of centrosomes and apicoplasts.

## Abstract

Toxoplasma gondii, an apicomplexan parasite with a broad host range, undergoes efficient proliferation through the unique endodyogeny during its tachyzoite stage, which is critical for triggering acute infections. As a specialized structure involved in T. gondii cell division, the centrocone and the regulatory mechanisms of its associated proteins remain incompletely elucidated. This study examined the functions of the centrocone protein SMC_N1 by constructing a conditional knockdown strain using CRISPR-Cas9 combined with the mini auxin-inducible degron (mAID) system. Immunofluorescence analysis (IFA) demonstrated that SMC_N1 exhibited periodic expression, with peak levels during the S phase and undetectable expression in the G1 phase. Depletion of SMC_N1 disrupted the integrity of inner membrane complex (IMC) assembly, the process of endodyogeny, and nuclear division. Additionally, SMC_N1 was involved in the stable inheritance of centrosomes and apicoplasts. In conclusion, this study revealed that the centrocone protein SMC_N1 is indispensable for the proliferation of T. gondii tachyzoites. Its unique expression pattern and functional specificity not only expand our understanding of the regulatory mechanisms underlying T. gondii cell division, but also provide a potential target for the development of novel anti-Toxoplasma drugs targeting parasite proliferation.

The highly efficient endodyogeny of tachyzoites is a key process driving acute infection by Toxoplasma gondii. The centrocone is a specialized and critical structure for parasite cell division, but the regulatory mechanisms of centrocone proteins in T. gondii remain poorly understood. In this study, we characterized the centrocone protein SMC_N1, which exhibited periodic expression in tachyzoites, peaking during the synthesis phase. Conditional depletion of SMC_N1 was achieved in the type I RH strain and type II cyst-forming PRU strain using the mAID system combined with CRISPR-Cas9. Depletion of SMC_N1 disrupted IMC assembly, endodyogeny and nuclear division, as well as the stable inheritance of the apicoplast and centrosome, resulting in severe defects in intracellular replication and impaired tachyzoite growth. Collectively, these results indicate that SMC_N1 regulates cell division by coordinating organelle inheritance and cytoskeletal dynamics, ensuring proper replication of T. gondii tachyzoites and provide insights into mechanisms controlling parasite proliferation.